Bi-directional torque generator devices are well known which are adapted to selectively rotate an output shaft in either a clockwise or counterclockwise direction. Usually, the output shaft of the device is connected to an input shaft of a driven device for rotation therewith, and rotation of the output shaft transmits torque to the driven device. In certain situations, however, the driven device may tend to back drive torque to the device. To prevent this back driving action, a back stopping clutch is usually coupled between the output shaft of the device and the input shaft of the driven device. A typical bi-directional back stopping clutch includes an input yoke connected to a pair of opposed brake shoes disposed within a hollow cylindrical brake drum. The brake shoes are generally semi-circular in shape, having arcuate braking surfaces which correspond to the inner surface of the brake drum. A pair of drive pins are disposed between the brake shoes, normally in parallel alignment. The drive pins extend into apertures formed in an output member, which is connected to the input shaft of a driven device.
Under normal operating conditions, the input yoke rotates the brake shoes, the drive pins, and the output member within the brake drum, thereby transmitting torque through to the driven device from the torque generating device. However, if the driven device attempts to back drive torque through the clutch, rotation of the output member causes the drive pins to become skewed. As a result, the brake shoes are spread apart from one another into locking frictional engagement with the inner surface of the brake drum. In this manner, the clutch prevents back driving of the torque generating device by the driven device.
The generally semi-circular brake shoes, which may include hardened wear-resistant inserts at the braking surfaces, are mounted in an opposing relationship. It is known in the prior art to form circular openings between the brake shoes at locations equally spaced on opposite sides of the axis of rotation for the input yoke. A pair of output pins extend between these openings and aligned openings on an output member. The output member is connected to an output shaft. These prior art shoe designs that utilize circular openings between the brake shoes require very tight tolerances, typically on the border of manufacturability, within the circular openings. As a result, a slight mismatch of pin radius and the circular openings formed between the brake shoes will prevent assembly or at least significantly compromise the clutch's performance. Additionally, the complicated geometry requires the use of castings which are then subjected to rough machining operations, increasing manufacturing time and expense.
Also in the prior art, the input yoke and the output member include "blind" holes requiring shaper cutting of key slots for both the input and output shafts. Again, tight tolerances are required for both the shaper cutting and the blind holes, and the shaper cutting keying process is very time consuming, difficult and expensive.
Prior art output pins have a generally cylindrically body with a quasi-spherical head at the end which fits into an output member opening. The spherical head allows the pin to tilt or skew in the output member opening sufficiently to engage the brake without interference between the skewed pins and the output member. The portion of the output pin between the brake shoes is formed with a uniform cylindrical body terminating at a flat end. Alternatively, it is known to construct an output pin having a generally spherical head and having two annular ridges, one adjacent the flat end of the pin and the other spaced between the two ends of the pin. In both cases, the generally cylindrical body of the pin engages the brake shoes within the generally circular openings formed between the brake shoes at locations equally spaced on opposite sides of the axis of rotation for the input yoke. A spring pocket is also formed in each brake shoe along the common border in a location between the circular openings formed between the brake shoes. The spring pocket requires machining a blind hole into each opposing brake shoe surface, and also requires tight manufacturing tolerances.
Thus, the manufacturing tolerances required for each respective component results in an overall total assembly stack-up tolerance. The total tolerance varies from one assembly to another. Further, the total tolerance can vary within each clutch assembly due to internal clearances of the parts. These differences can lead to a substantial variation in clutch performance between different assemblies.